Coverage represents a primary requirement of cellular radio communication systems. For example, the desired service coverage and quality sets limits on how sparsely network radio nodes can be deployed and where they can be deployed. Coverage requirements therefore directly affect deployment costs.
In the Long Term Evolution (LTE) standard defined by the Third Generation Partnership Project (3GPP), coverage is an important design parameter. For example, voice services retain their continued importance in LTE networks, where voice services typically use equal (data) rates in both the uplink and the downlink. However, because of the lower transmit power limitations of typical mobile devices, the uplink typically limits coverage.
Release 8 (Rel. 8) of the LTE standard introduced Transmission Time Interval (TTI) bundling as a technique for improving voice coverage using Voice-over-IP (VoIP) services. Rather than relying on Hybrid Automatic Repeat reQuest (HARQ) processing to trigger retransmission of unsuccessfully received VoIP packets, TTI bundling allows a mobile device to send the same packet over a “bundle” of consecutive TTIs, e.g., four TTIs. More particularly, the mobile device sends the same packet in each one of the bundled TTIs, but uses a different error detection and correction coding in each TTI, thus maximizing the ability of the network to successfully recover the packet from the bundled transmission.
Therefore, rather than relying on HARQ processing, with its attendant signaling overhead, a base station may activate TTI bundling for a given voice user, responsive to recognizing poor uplink reception conditions with respect to that user. In more detail, with TTI bundling, four uplink TTIs are bundled together using autonomous retransmissions. This technique effectively reduces the overhead of the transmissions, because the same header information is used in all bundled TTIs, rather than applying separate headers. The channel coding efficiency is also increased because of the use of longer code words.
With TTI bundling, an uplink grant is sent to a device (i.e., a UE or other wireless communication device being scheduled by the base station) four ms before the start of the bundle. The device then transmits one transport block using one redundancy version per subframe over four subframes. Correspondingly, the device expects HARQ feedback sixteen ms after the initial grant. If a negative feedback is received a non-adaptive retransmission is performed four ms after the feedback. For further TTI bundling details, the interested reader may refer to the 3GPP Technical Specification (TS) 36.321.
While TTI bundling is an efficient way for improving coverage for coverage-limited VoIP services, TTI bundling is not without certain drawbacks—see the 3GPP study item identified as R1-120900, “Way Forward on Uplink Coverage Enhancement,” RAN 1#68 Dresden February 2012. One particular drawback arises because the most commonly used VoIP codecs generate one new data frame every twenty ms.
These twenty ms frames should be conveyed to the (network) receiver with the lowest possible delay. The allowed air interface delay for LTE is fifty ms, which limits the number of HARQ retransmissions available, given the maximum HARQ Round Trip Time (RTT) of sixteen ms. This circumstance does not permit single transport blocks to be used for the twenty ms frames, which lowers coverage to the case where a single transport block could be used for each twenty ms frame.
Multiple solutions have been proposed to improve the TTI bundling scheme of LTE Rel8 to avoid the above problems. Among these various proposals is the proposal that more TTIs be bundled together. This approach provides additional scheduling flexibility.